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I forget the details, but I thought iMovie could do this.

If not, QuickTime Pro can do some splices with cut/copy/paste, and it's only $30.
 
You may already have it on your hard drive. iMovie, part of Apple's iLife suite, comes pre-installed on most Macs (including the mini) is right there ready to go. You can import, edit, and combine movie files into one large movie.

VLC is a freeware player that can play your existing movies back-to-back using it's playlist feature without the need to combine them into 1 large movie
 
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